Optical scanners operate by imaging an object (from a sheet of paper, document or other form of medium) with a light source, sensing a resultant light signal with an optical sensor array, and each optical sensor in the array generating a data signal representative of the intensity of light impinged thereon for that portion of the imaged object. The data signals from the array sensors are then processed (typically digitized) and stored on a suitable medium such as a hard disk of a computer for subsequent display and/or manipulation. The image of the scanned object is projected onto the optical photo sensor array incrementally by use of a moving scan line. The moving scan line is produced either by moving the document with respect to the scanner optical assembly, or by moving the scanner optical assembly relative to the document. Either of these methods may be embodied in flat bed scanners, hand held scanners, or any scanner having automatic document feed capabilities.
Various types of photo sensor devices may be used in optical scanners. For example, a commonly used photo sensor device is the charge coupled photo sensor device (CCD). A CCD builds up an electrical charge in response to exposure to light. The size of the electrical charge built up is dependent on the intensity and the duration of the light exposure. In optical scanners, CCD cells are aligned in linear arrays. Each cell or "pixel" has a portion of a scan line image impinged thereon as the scan line sweeps across the scanned object. The charge built up in each of the pixels is measured and discharged at regular "sampling intervals." In most modern optical scanners, the sampling intervals of the CCD arrays are fixed.
As previously mentioned, an image of a scan line portion of a document is projected onto the scanner's linear sensor array by scanner optics. In CCD scanners, the scanner optics comprise an imaging lens which typically reduces the size of the projected image from the original size of the document considerably. Pixels in a scanner linear photo sensor array are aligned in a "cross" direction, i.e., a direction parallel to the longitudinal axis of the scan line image which is projected thereon. The direction perpendicular to the "cross" direction will be referred to herein as the "scan" direction (i.e., paper or scanner movement direction for scanning of the image).
At any instant when an object is being scanned, each pixel in the sensor array has a corresponding area on the object which is being imaged thereon. This corresponding area on the scanned object is referred to herein as an "object pixel" or simply "pixel." An area on a scanned object corresponding in area to the entire area of the linear sensor array is referred to herein as an "object scan line" or simply "scan line." For descriptive purposes a scanned object is considered to have a series of fixed adjacently positioned scan lines. Further, scanners are typically operated at a scan line sweep rate such that one scan line width is traversed during each sampling interval.
Differentiating from scanners employing CCDs, a contact image sensor (CIS) and CIS drive roller are commonly employed in document fed scanners for imaging the medium being passed (fed) through the scanner. The CIS is spring loaded against the drive roller and forms a nip therebetween. The medium being scanned is presented for scanning at the nip and is pulled passed the CIS by the drive roller. The CIS typically comprises a glass plate adjacent the roller (forming the nip), an array of light sources such as light emitting diodes (LED's) directed at the nip, an array of self-focusing lenses (cylindrical microlenses) that direct and focus the light from the light sources as reflected off the medium (or roller if no medium is present), and an array of photo sensors adjacent the self-focusing lenses for converting the light passed through the lenses to electrical signals for processing of the image generated. An advantage of the CIS is that it is less susceptible to having foreign particles (i.e., dust) settle on the CIS optics which could degrade the scanned image quality. A CIS is less susceptible to foreign particles because it has fewer reflecting optics, relative to CCD scanner devices, for focusing the light. Another advantage of the CIS is its small size due to its optical configuration.
Certain document fed scanners employ an automatic document feeder (ADF) for automatically feeding the document (medium) through the scanner. ADFs typically employ sensors or multiple sensors for detecting a leading edge of the document as it is automatically grabbed and fed into the scanner by a "pick"-roller (or "D"-roller). From the point in time that the sensors detect the leading edge, the number of steps (or amount of time) needed to transfer the leading edge to the imaging assembly, such as a contact image sensor (CIS), are monitored so that the imaging assembly may initiate actual imaging at the precise time when the leading edge of the medium reaches the CIS.
Certain drawbacks exist with this conventional approach. For example, with mechanical sensors, most printers can "repeat" only from about 1/8 to 1/4 inch top-of-page--meaning, even with the system working properly, actual initiation of image scanning relative to the actual top-of-page may vary 1/8 to 1/4 inch from page to page. This is due to factors such as media type, temperature and humidity. Furthermore, scanners are generally configured to scan various types and sizes of documents having differing weights and composed of differing materials (for example, sheet paper, photographs, transparencies, etc). As such, "slip" may occur between the document and the pick-roller as the document is grabbed and transferred to the imaging assembly. Once the document reaches the imaging assembly, it is passed through the nip formed between the imaging assembly and the drive roller. Prior to reaching the nip (and thereby being pulled by the drive roller), the amount of slip may be highly variable, depending upon several factors related to the document being handled, such as stiffness, surface finish, cross sectional area, etc. (with these also being a function of temperature and humidity). CCD scanners don't typically have as much slip problem as CIS scanners because there is no pressure formed at the imaging location of the CCD scanner optics assembly (i.e., there is no driver roller forming a nip). In contrast, there is pressure at the nip of a conventional CIS optics assembly because of the pressured drive roller forming the nip with the CIS assembly.
Since the amount of slip can vary in CIS scanners, and since actual imaging is, conventionally, only initiated at the imaging assembly based upon the number of steps counted (or amount of time detected) from the time the leading edge passes the sensors, there can be a highly variable difference in the top of the image scanned (relative to the document top-of-page). For example, it has been shown that about a one inch slip difference can occur between a photograph and a piece of sixteen (16) pound paper. This, of course, variably offsets the top of the image scanned for each document.
In certain prior art auto-feed imaging systems it has been known to increase the pick-roller force to reduce slip of the document. However, this method tends to increase "buckle" problems for the document in the transfer path (because of the nip pressure), and also severely limits the range of types of media that can be used by the imaging system (i.e, thicker documents have more trouble at the pick roller).
In systems that do not use ADF devices, "slip" does not occur because there is no pick-roller and the operator manually pushes the document into the system (to the nip between the CIS and drive roller).
In other prior art CIS systems it has also been known to use black driver rollers so that white (or lighter colored) media can easily be detected for initiating actual imaging of the document. However, this limits the type and color of media that can be used and/or detected well in the system. Specifically, a true edge of the document being imaged cannot always be detected because a change from black is what is being sensed. As such, if a black border document is scanned, the document's true edge will not be detected.
Another problem with the black roller is that in certain imaging systems, the image scanned can be sent directly to a printer device from the scanner for immediate printing (as a "convenience copy") without going through a host computing device for data manipulation options. In the event a black roller is used and a "convenience copy" is selected for imaging something smaller than the full document size, then a black border will surround the scanned image. For example, if a small business card is scanned, most of the page of the convenience copy will be black border surrounding the imaged business card. This is especially undesirable because of the excessive and wasteful use of toner in the printer device as a result of producing the black border.
Yet another problem associated with scanners that provide a convenience copy is if the actual edge of the medium is not properly detected so that page/image dimensions are accurately determined, then the convenience copy may very well end up being "two" copies because a one page original is "seen" (scanned) as extending beyond a single page. Thus, two pages are printed, each with part of the original image scanned thereon. This problem is also compounded given that laser printers, for example, reserver a border area around the page as a non printable area. As such, if the actual top-of-page is not detected, and scanning initiates late, the first scanned area will be deemed the "border" area and "thrown out" during a convenience copy process.
Given the forgoing problems and limitations of prior art image scanning methods and apparatus, objects of the present invention are to provide an improved means and method for accurately sensing a top-of-page for improved imaging in a scanner, and especially in a scanner having automatic document feed capabilities in connection with CIS optics.